REPAIR GUIDE - TurboExpress/PC Engine GT: Video Fix

[BlueBMW]

You should be able to pick up a cheap one at sears for less than $20 also.

[Duo_R]

I don't hardly recommend this, but if you are on a budget and have a store near you would be worth it just to check out your problem. just be warned it is a cheepie.

http://www.harborfreight.com/7-function-digital-multimeter-92020.html

I bought a cheap one at Harbor Freight.

[Charlie]

As an FYI, there are already two places on the board that are ideal spots to connect the new capacitor.   To find them, draw (mentally, of course) a line between the center of  CC500 and CC501.  The location is half way down that line, almost dead center between the two caps.  Note that one is a hole; the other is a TestPoint that  is covered with solder. But it's still a good mechanical connection.

The positive of CC501 goes to the hole close to Q500, the negative goes to the test point close to D502.
Based on the picture above, it looks like you have to dig out the latex to see them!

Picture showing open hole (positive connection) and test point(negative connection):


Charlie


(Edited 12/1)

[ApolloBoy]

Give that a shot, swap screens on both units and see what happens   

I finally got around to swapping the guts between my friend's TE and my TE (which was easier than swapping the screens, and the screen is not the issue. Something must be seriously wrong here since I've tried everything to get the video working on this unit.

[cornacchia]

Hey everyone, so ApolloBoy tried to fix my turboexpress and could not get it going.  I was wondering if anyone else would be willing to look at it and see if they could do anything.

[BlueBMW]

I dont mind giving it a shot.  Let me know   Or alternately, chop5 is a magician when it comes to this stuff

[nat]

Rather than start a new topic, I thought I'd post this here. Question mainly aimed at Charlie, since he seems to have these PCB layouts down pat:

I've got a GT PCB that has an odd problem. Something on the board is causing the LCD backlight to "pulsate" while the unit is on. Pulsation happens the same on both battery and AC power. It's really quite distracting while playing, and I'd like to isolate the cause but so far have been unable. The problem is definitely on the PCB itself rather than in the LCD/backlight assembly, as I have several TurboExpress and PCE GT units that I've swapped the PCB into and the problem definitely follows the PCB.

I haven't replaced all the capacitors on the PCB, but I've replaced all the ones I know to be related to the video. Doesn't mean there aren't others that I'm unaware of, however.

Ideas?

[Charlie]

Typically, the pulsation occurs because of:
1. inadequate power.  on batteries, you may have dead batteries (DUH!), but on AC you should not have that with an adequate power supply.
Make sure the power supply you are using has enough current to handle the TE.  I'd go at least 20% above the TE specs.
(Edit: you can often tell this is the problem if the pulsation occurs more strongly when any loud sound effects occur.  Turn the volume up full and see if you can see a correlation between the pulse and the audio)
2. bad cap (or possibly other part) on the display high voltage line.  The high voltage is some 200+VAC generated by an inverter circuit.  
This circuit needs not only a really heavy voltage source, but a really "clean" (ie, noise-free and ripple-free) source as well.

We'll need a VOM or 'scope to go further.

Charlie

[nat]

Typically, the pulsation occurs because of:
1. inadequate power.  on batteries, you may have dead batteries (DUH!), but on AC you should not have that with an adequate power supply.

This problem has been going on for a number of years, through countless sets of batteries.

Make sure the power supply you are using has enough current to handle the TE.  I'd go at least 20% above the TE specs.

I'm  using the OEM TurboExpress AC adapter. Works fine/without issue with the other GT/TE units I have.

(Edit: you can often tell this is the problem if the pulsation occurs more strongly when any loud sound effects occur.  Turn the volume up full and see if you can see a correlation between the pulse and the audio)

Will do... This I have not done.

2. bad cap (or possibly other part) on the display high voltage line.  The high voltage is some 200+VAC generated by an inverter circuit.  
This circuit needs not only a really heavy voltage source, but a really "clean" (ie, noise-free and ripple-free) source as well.

We'll need a VOM or 'scope to go further.

I've got my VOM ready!

Also, one thing I noticed I failed to mention... It only happens after the the unit warms up. The first 30 minutes of play are pulsation-free. This is what points me in the direction of a bad capacitor somewhere....

[Charlie]

There are either 4 or 5 caps in the inverter circuit.  Plus a coil, a transformer, a transorb/PTC, and a few other things I don't remember.  Yes, caps is the obvious choice.

Three other items....
1. Does the pulse change if you tap or bang on the PCB, particularly in the inverter circuitry?
2. Does the whole screen pulse at once, or does the pulse "travel" in a wave up/down the screen?
3. Does playing with the contrast control make any difference to the pulse? Obviously, it WILL affect the display contrast overall!

Charlie

[nat]

There are either 4 or 5 caps in the inverter circuit.  Plus a coil, a transformer, a transorb/PTC, and a few other things I don't remember.  Yes, caps is the obvious choice.

Three other items....
1. Does the pulse change if you tap or bang on the PCB, particularly in the inverter circuitry?

Will get back to you....

2. Does the whole screen pulse at once, or does the pulse "travel" in a wave up/down the screen?

The whole screen. It seems like it's the backlight that is pulsating and not the image itself, although I could be wrong.

3. Does playing with the contrast control make any difference to the pulse? Obviously, it WILL affect the display contrast overall!

No, aside from the intended effect of using the control.

[Charlie]

At a quick glance, active caps are CC501, C502, C501, C500,C504....but I don't think all of them are electrolytic.

Also involved: D502, Q500, R500, L500, DZ500, Q501, D503...but I would put them at a lower expectation of failure.

You might want to check the backlight voltage at the connector, see if the VOM shows a fluctuation that matches the pulses.  Probably yes.

Charlie

Edit: Also involved CC900, C901, CC901, T900, L900, D902.

[thesteve]

although unlikely a failing lamp can cause pulsation, typically after warmup.

[nat]

Like I said, the problem follows the logic board. I moved the logic board from one GT to another GT (with a completely different backlamp) and the issue follows the board.

[thesteve]

ceramic cap failures are not unheard of in inverter circuits.
did the problem occur with or without use.
if it developed the prob on the shelf it will be electrolytic caps, due to drying.